1. Field of the Invention
The present invention relates to a method and system for removing with parison an article from a blow molding machine such as a machine for blow molding containers for detergents, decorative bottles or the like.
Also, the present invention relates to a method for controlling the length of a parison in a blow molding machine.
2. Description of the Related Art
(A) Article Removing Method
Japanese Unexamined Patent Publication No. 50-111167 discloses a rotary blow molding machine for sequentially molding hollow resin articles employing a thermoplastic resin parison. Such rotary blow molding machine has a pair of rotary frames. Between the pair of rotary frames, a plurality of pairs (8 pairs) of molds which can be opened and closed by means of a locking mechanism, are disposed. Though an extruder positioned above the pair of rotary frames, a parison is supplied to each pair of molds positioned beneath the extruder sequentially on a one-by-one basis. After locking the mold the parison is sheared off the extruder by rotation of the pair of frames in a predetermined direction. After blow molding, each blow molded article is removed from the pair of opened molds by chucking flash thereof by a chuck of an article removing device.
However, in the above-mentioned conventional rotary blow molding machine, the parison supplied to the pair of molds is apt to be sheared off the extruder upon rotation of the pair of rotary frames. The flash at the upper end of the parison, which extends upwardly from the upper surface of the pair of molds, tends to fall down to the upper surface of the pair of molds so that it may be solidified in the fallen position to become the flash of the blow molding article. In such case, difficulty has been encountered in chucking such flash by the chuck of the article removing device.
Also, since the articles removed by the article removing device are discharged to downstream processes for quality discrimination, it is possible that defective articles will be supplied to the downstream processes.
(B) Parison Length Control Method
The blow molding machine performs blow molding by the processes of downwardly extruding the parison from the extruder, closing the end of the parison, injecting a blowing gas into the closed parison to expand the parison it has the configuration of the cavity.
To this end, the length of the parison extruded from the extruder has to be controlled to a constant length for the reasons 1.about.3 set out below.
1 The thickness of the parison extruded from the extruder is cyclically controlled at a regular interval toward a target parison length in order to provide desired thickness for the articles after blow molding. Accordingly, in order to obtain high precision thickness distribution of the article with the cyclically performed parison thickness control, it becomes necessary to accurately maintain a constant parison length at the time of closing the molds. PA1 2 When the length of the parison is excessive, the length of the lower end of the parison protruding from the molds becomes excessive, causing substantial loss of material. PA1 3 When the parison is too short, it becomes difficult to clamp the lower end of the parison to the molds and makes blow molding impossible. Therefore, in the prior art, proposals were made for controlling parison length, as disclosed in U.S. Pat. No. 3,759,648 and Japanese Unexamined Patent Publication No. 51-41061. PA1 1 In both U.S. Pat. No. 3,759,648 and Japanese Unexamined Patent Publication No. 51-41061, the relationship between the object to be monitored (parison length) and the variable to be controlled (screw rotation speed of the extruder or the clamping timing) is so indefinite that the control constant has to be determined on a trial-and-error basis. Therefore, it takes a long time to determine the optimal value. Furthermore, reproductivity of the control is rather poor. PA1 e,crc/2/ In both U.S. Pat. No. 3,759,648 and Japanese Unexamined Patent Publication No. 51-41061, discrimination is made only as to whether the current parison is too long or too short, ignoring the offset magnitude from the target value (parison length difference). Therefore the control, depending upon parison length difference, cannot be performed to maintain precision level of control. In addition, when the offset magnitude is relatively small, it often causes hunting of the control and when the offset magnitude is relatively large the control takes a relatively long time. PA1 3 In Japanese Unexamined Patent Publication No. 51-41061, since the variable to be controlled is the timing of clamping, difficulty is encountered in synchronization of operation with the parison thickness control which is performed cyclically at a regular interval. This tends to cause degradation of precision of thickness distribution in the article. Furthermore, modification of clamping timing causes necessity of adjustment of operation timing and/or operation speed of the mechanical system and accordingly reduces the efficiency of the molding operation. PA1 correcting the configuration of the upper end of the parison into a flat and vertically extending configuration before solidification; PA1 performing quality discrimination for discriminating a normal article from a defective article during a step between the stage of initiation of extrusion of the parison and the stage of removing the article; and PA1 chucking the upper flash at an article removing stage to discharge the removed articles to a normal article discharging destination for the normal article and alternatively discharging defective articles to a defective discharging destination on the basis of the result of the quality discrimination. PA1 (a) means for Judging whether the parison is defective upon initiation of molding operation; PA1 (b) means for judging whether the parison is defective when its length is outside a target length range; PA1 (c) means for detecting blowing gas supply failure during parison blowing; PA1 (d) means for detecting failure of downstream facility following the blow molding machine; PA1 (e) means for detecting absence of an article at the article removing station; PA1 (f) means for detecting when lower flash of article is not present at a predetermined position at the article removing station; PA1 (g) means for detecting abnormality when the article removing device erroneously holds an article; and PA1 (h) means for detecting defects in articles molded by molds on which the absence of an article is detected in the above-mentioned item (e) in preceding cycle. PA1 a flash configuration correcting device provided above the mold and arranged to shape the upper end of the parison into a flat and vertically extending configuration before solidification of the parison; PA1 an article removing device arranged to clamp an upper flash of the molded article and to remove the article from the mold; and PA1 a defective discrimination control device for performing quality discrimination for discriminating between a normal article and a defective article during the process between a step of initiation of extrusion of the parison and the step of removing the article and controlling the article removing device to discharge the removed article to a normal article discharge destination for the normal articles and to a defective discharge destination for the defective articles, all on the basis of the results of quality discriminations. PA1 an article removing device provided above the mold and shaping the upper end of the parison into a flat and vertically extending configuration before solidification of the parison, and clamping an upper flash of the molded article and removing the article from the mold; and PA1 a defective discrimination control device for performing quality discrimination for discriminating a normal article and a defective article during a process between initiation of extrusion of the parison and removing the article and controlling the discharge of the removed articles to a normal article discharging destination for the normal articles and to a defective discharging destination for the defective articles, all on the basis of the result of the quality discrimination. PA1 (1) Since the shown embodiment shapes the upper end of the parison extending upwardly from the upper surfaces of the molds into a flat and vertically extending configuration before solidification, the upper flash can be certainly and readily chucked. Accordingly, removal of the articles can be assured and a continuous operation performed without stopping operation throughout initiation of molding to the normal operating condition. PA1 (2) By discriminating between normal articles and defective articles during the stage between starting extrusion of the parison to removal of the article, normal articles are discharged to the destination for the normal produce (normal article discharging conveyer) and the defective articles are discharged to the destination for the defectives (defective discharging conveyer). Accordingly, not only in the molding process, but also in the article removing process, quality discrimination between the normal articles and the defective articles can be performed to accurately discharge the normal articles and the defective ones separately. PA1 (3) By the above-mentioned effect (1), the defective articles upon initiation of the molding operation can be automatically discharged without stopping the machine. This enables automating the initial operation. PA1 (4) By the above-mentioned effects (1) and (2), the defective articles in normal operation can be automatically taken out without stopping the machine to eliminate the necessity of re-starting the operation associated with stopping of the machine, and avoiding discharging of the defective articles to the downstream processes. Therefore, sequential molding can be achieved. PA1 preliminarily deriving a target relationship between a parison length of a resin to be used and the screw rotation speed of the extruder depending upon the resin to be used, during a test molding process; PA1 detecting a current parison length during normal molding operation; PA1 deriving a parison length difference between the current parison length and the target parison length; PA1 controlling the extruder screw rotation speed by calculating a correction amount for the extruder screw rotation speed according to the preliminarily determined relationship between the parison length and the extruder screw rotation speed to attain the target parison length. PA1 (1) The control constant is calculated automatically. Accordingly, the control constant can be derived quickly without relying on the operator's trial and error, and thus provides good reproducing ability of the control. PA1 (2) Since the control constant can be derived corresponding to the resin, it may provide high precision of control. PA1 (3) Since the parison length is controlled on the basis of the error (parison length difference) between the current parison length and the target parison length, high precision can be provided for control. Also, since the control is performed depending upon the parison length difference, possibility of causing hunting can be reduced and the converging period can be shortened. PA1 (4) Since the object of control is the extruder screw rotation speed and need not modify the timing of locking of the molds, synchronization to the parison thickness control which is performed cyclically with a constant interval can be facilitated. In addition, since the parison length can be controlled to be constant, the thickness distribution of the article can be controlled with high precision. PA1 (5) Since the locking timing of the molds is not necessarily modified, it is unnecessary to adjust the operation timing and the operation speed of the mechanical system to permit adaption for high speed molding. PA1 (6) The deadband can be controlled on the basis of the data reflecting the fluctuation of the current parison length with respect to the molding device to be actually used. Therefore, the deadband can be controlled to the proper range to avoid hunting. PA1 (7) The deadband can be calculated automatically. Accordingly, the deadband can be set quickly without requiring trial and error of the operator. Also, high reproducing ability can be achieved. PA1 (8) The current parison length is calculated from the light blocking period, making it possible to certainly and easily derive the parison length. PA1 detecting a current parison length during normal molding operation; PA1 deriving a parison length difference between the current parison length and a target parison length and deriving a relationship between the parison length of a resin to be used and the screw rotation speed of the extruder; and PA1 controlling the extruder screw rotation speed by calculating a correction amount for the extruder screw rotation speed according to the determined relationship to attain the target parison length. PA1 (1) The control constant is calculated automatically. Accordingly, the control constant can be derived quickly without relying on the operator's trial and error, and thus provides high reproducing ability of the control. PA1 (2) The control constant can be derived with respect to the current used resin, and especially depending upon the property variation of the resin in real time. Accordingly, high precision control can be achieved. PA1 (3) Since the parison length is controlled on the basis of the error (parison length difference) between the current parison length and the target parison length, high precision can be provided for control. Also, since the control is performed depending upon the parison length difference, possibility of causing hunting can be reduced and the converging period can be shortened. PA1 (4) Since the object of control is the extruder screw rotation speed and not the timing of locking of the molds, synchronization to the parison thickness control which is performed cyclically with a constant interval can be facilitated. In addition, since the parison length can be controlled to be constant, the thickness distribution of the article can be controlled with high precision. PA1 (5) Since the locking timing of the molds is not modified, it is unnecessary to adjust the operation timing and the operation speed of the mechanical system to permit adaption for high speed molding. PA1 (6) The deadband can be set on the basis of the data reflecting the fluctuation of the current parison length with respect to the molding device to be actually used. Therefore, the deadband can be set at the proper range to successfully avoid hunting. PA1 (7) The deadband can be calculated automatically. Accordingly, the deadband can be set quickly without requiring trial and error of the operator. Also, high reproducing ability can be achieved. PA1 (8) The current parison length is calculated from the light blocking period, so that it becomes possible to certainly and easily derive the parison length.
The method disclosed in U.S. Pat. No. 3,759,648 performs discrimination whether the parison length is sufficiently long or too short by checking whether or not the parison reaches a position to block a light beam of a phototube unit. This is done with specific timing with respect to the clamping timing. Based on the results of this discrimination the screw rotation speed of the extruder is modified to ensure a given length of the parison in accordance with the clamping timing.
The method disclosed in Japanese Unexamined Patent Publication No. 51-41061 includes two sets of phototube units arranged below the extruder so as to discriminate regarding the length of the parison in such a manner that (a) when the light beams of both phototube units are not blocked by the parison, the decision is made that the parison length is too short, (b) when the light beam of only the upper phototube unit is blocked, the parison length is proper, and (c) when the light beams of both of the phototube units are blocked, the parison is too long. Based on these results the clamping timing is modified so as to ensure a constant length of parison in accordance with the clamping timing.
However, the prior art set forth above encounters the following problems 1.about.3.